Unless you are into having just one large fish in a very large tank then i would recomend staying well clear of black sharks, once they reach over 12" (and they grow very fast) they become hyper aggressive and will attack and kill pretty much anything in the tank, i know this because i had one and had to return it to a store within 9 months, some people have had success with keeping much smaller fish such as giant danios and medium sized barbs with them but for most its the solitary life. They will get to at least 2 feet long and are a very heavy thick set fish so you need a very large tank to house an adult, something along the lines of a 8x3x3 with a big sump style filter to deal with the huge ammount of waste the fish produces would be the absolute minimum.